Prefix for structure or red (5)

I believe the answer is:

infra

'prefix for structure or red' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for infra that I've seen before include "-- Dig means degrading" , "Coming below" , "Below (as prefix)" , "lower down" , "Dig is demeaning" .)

I've seen this clue in the King Feature Syndicate and the Wall Street Journal.

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